The invention pertains to a cured epoxy resin composition suitable for protecting the surfaces of sheet materials such as metals. More particularly, the invention is directed to applied layers of a material containing apolyepoxide and a Mannich base of a hydroxystyrene homopolymer or copolymer.
U.S. Pat. No. 4,399,257 teaches poly-p-vinyl phenol as a hardening agent for epoxy resins in the presence of a Sn salt of an organic acid. U.S. Pat. No. 4,442,246 teaches a self-emulsifiable ester of epoxy and an acid containing addition polymer.
The foregoing patents indicate the desire for applying protective coatings to metal and other surfaces for the purpose of enhancing resistance to corrosion. Many coating compositions directed to this end are well documented in the published literature and many are commercially available as well. Typical prior art protective coatings include pigments suspended in a vehicle. The vehicle consists primarily of a resinous binder dissolved in solvents or dispersed in water together with small quantities of driers, plasticizers, and stabilizers as required by the intended end use. As the film dries the vehicle converts into a solid film either by evaporation of solvents or water, oxidation or polymerization through the application of heat, addition of a catalyst or a combination of reactive components. Protective coatings include oleoresinous paints or drying oils, oil based paints, and alkyd phenolic, epoxy, chlorinated rubber silicone, vinyl, acrylic, nitrocellulose, polyester and polyurethane paints and varnishes. The selection of coating depends on its intended application and the degree of protection required for the end use conditions.
In known prior art epoxy coatings, the resins consist of a polymerization product of epichlorohydrin and bisphenol A. This resin then undergoes a cross-linking reaction on the metal surface through the action of a curing agent and optionally through the application of heat to accelerate the cure. The catalyst employed is chosen depending on the coating properties desired.
Polyamine curing agents produce hard, high gloss, chemically resistant coatings for interior or exterior atmospheric exposure. The disadvantages of polyamine curing agents are the necessity for accurate measurements of small quantities and the toxic nature of the curing agent. These polyamines contain a multiplicity of --NH.sub.2 and --RNH groups and are known as curing agents. Amine "catalysts" are generally tertiary amines which promote epoxy/epoxy reactions.
Amine adduct curing agents are polyamines partially reacted with less than an equivalent amount of the epoxy resin. These curing agents are relatively nontoxic.
Polyamide catalysts are polyamines reacted with unsaturated fatty acids, usually dimerized linoleic acid, in ratios designed to yield amine terminated polyamides.
The polyamine or polyamide curing agent is added to the epoxy resin just prior to application. This causes a crosslinking of the molecules and curing of the coating after it has been applied to the surface.
Cured epoxy coatings show excellent resistance to a wide range of chemicals, solvents, oils, acids, and alkalies. In addition, they exhibit excellent adhesion to almost all types of surfaces, including metal, wood, and concrete.
However, basic amine curing agents may require special care with respect to health hazards in addition to normal safety precautions.
This invention provides a coating containing Mannich bases of poly(p-hydroxystyrene) and copolymers thereof which are used to cure epoxy resins. The system may be water-borne or solvent-borne.
A prospective advantage to using the Mannich base as a curing agent is that the tertiary amine functionality may act as a built-in catalyst for the phenolic hydroxyl/epoxy curing reaction thus reducing or eliminating the need for the addition of an external catalyst, although external catalysts may be used in the composition.